Welding is a process of joining metals by melting workpieces to form a molten “weld puddle” that coalesces upon cooling to form a metal joint. Often pressure and/or ultrasonic energy are also applied to the components as well as heat in order to produce a weld. In the context of microelectronics, the specialized welding process most often used is termed “wirebonding”. Wirebonding generally refers to the process of forming an electrical connection between the silicon chip and the external leads of the semiconductor device using very fine wires welded to terminals at each end, such as a bond pad on a chip and a lead finger on a leadframe. The wires used in wirebonding are usually made of gold. There are two common wirebond processes distinguished by the shape of the weld, “ball” bonding, and “stitch” or “wedge” bonding. During wirebonding, a molten “free air ball” is formed by melting the free end of the wire, held by a “bonding head” or “capillary”, using a high voltage spark in a technique known as electronic flame-off (EFO). The free air ball typically has a diameter ranging from 1.5 to 2.5 times the wire diameter. The free air ball is then brought into contact with the bond pad, which is usually heated by other means. The general practice in the arts is to apply pressure to the press the free air ball into the surface of the bond pad. To further assist in ensuring adequate contact between the bondwire and bond pad, it is common to also apply ultrasonic forces to the bond wire, in effect rubbing it in contact with the bond pad during heating to form a metallurgical weld between the ball and the bond pad and deforming the weld itself into its final shape. Continuing the process, the wire is run from the chip to a corresponding terminal such as the finger of a leadframe, forming a gradual arc or “loop” between the bond pad and the leadfinger. Again, EFO, pressure, and ultrasonic forces are applied to the wire to form a stitch bond with the leadfinger. The wirebonding machinery severs the wire in preparation for the next wirebond cycle, usually by clamping the wire and raising the bonding head. The cycle repeats with repositioning the wire for a new bond. It is known to make numerous such wirebond connections between bond pads on a chip and the ends of leadfingers, often at the rate of several per second.
Although wirebonding is well known, problems persist in forming satisfactory wirebonds due to a variety of factors. One of the problems is the potential to cause damage to the bond pads due to the application of pressure and/or ultrasonic energy. Particularly in the case of delicate bond pads having multiple layers including metallic and low-K or ultra-low-K layers, the ultrasonic energy can cause cracks in one or more layers, which may inhibit or prevent the formation of a weld, or may weaken the bond pad making it prone to further breakage later. Contaminants or irregularities on the bonding surface can also inhibit weld formation. In addition to potential problems with weld formation, another related problem is “ball lifting” after weld formation due to defective welds. The formation of strong welds may be inhibited by the presence of contaminants on the bond pad, which act as barriers between the ball and the bond pad. Common contaminants that inhibit wirebonding may include residual glass, photoresist, silicon dust, die attach adhesive, or other impurities generated during the manufacturing process. Corrosion, or the formation of metal oxides on the exposed surface of the bond pads or lead fingers may also prevent the formation of an adequate weld. Yet another potential for problems may arise from disturbed or uneven bond pad surfaces. Such irregularities in the bonding surface may cause unexpected variations in the weld formation process. Excessive interdiffusion between the bond pad and wirebond metals or the formation of voids underneath the bond may also arise to create weak, and ultimately lifted welds. There is a need in the arts for improved wirebonding methods and systems for ensuring uniformity in the welding process and the formation of robust welds while reducing or eliminating one or more of these and possibly other problems.